1. Field of the Invention
The present invention relates to the generation of low energy ion beams with particular emphasis for use in implantation of ions into semiconductor materials
2. Description of the Prior Art
The production of low energy ion beams has increased in importance in recent years, and a significant amount of effort has been expended to enhance the low energy capabilities of ion beam systems, particularly those used in ion implantation.
In FIG. 1, a generic ion beam generation system is shown where an ion beam extraction system (comprising an ion source 1 and an extraction electrode 2) is followed by a sector analyzing magnet 3 and a target 4. The principal approach to increase low energy ion beam production has been to increase the ion beam output from ion sources with low extraction voltages. To produce 10 keV .sup.11 B+ ions from a plasma operating with boron trifluoride gas (BF.sub.3), the extraction supply would be set to 10 kV. Because of the space charge limit law and other limitations, this approach has achieved only moderate success to date.
An alternative approach is to extract molecular ions from the ion source and to allow these ions to impact upon a target. For example, if one desired to produce a low energy beam of boron ions, one may operate the ion source in FIG. 1 with boron trifluoride gas (BF.sub.3). A substantial quantity of BF.sub.2 + ions may be produced and extracted from typical ion sources such as, for example, a Freeman ion source or hot filament Penning Ion Gauge (PIG) source. This molecular ion may then be accelerated, selected by the magnet and impact into the target. Upon striking the target, the BF.sub.2 + ionic molecule will dissociate into boron and fluorine atoms. These components of the molecule will have effective energies, E.sub.Effective : EQU E.sub.Effective =E.sub.Molecule *(M.sub.Atom /M.sub.Molecule)(1)
where E.sub.Molecule is the energy of the molecule M.sub.Atom is the mass of the atom, and M.sub.Molecule is the mass of the molecule.
For the boron ion derived from the BF.sub.2 molecule, the boron ion will have: EQU E.sub.Boron =E.sub.BF2 *(11/49). (2)
Thus, to obtain a boron atom with an energy of 10 keV, the BF.sub.2 molecule would be extracted at 44.55 kV. Significantly higher beam currents may be achieved at this extraction voltage than at the 10 kV extraction voltage used for direct boron extraction from the ion source.
However, a limitation of the molecular ion technique at present is that undesired atoms will impact into the target. In the example above, the fluorine atoms will become resident in the target. This contamination may be unacceptable in a number of processes.